Polarization control is a significant concern in the design and operation of integrated optical devices. Planar waveguides pose an especially difficult problem for the designer who requires polarization-independent behavior.
In a waveguide, which is generally characterized by an effective index of refraction, neff the two polarization modes, quasi-TE (Transverse Electric) and quasi-TM (Transverse Magnetic), will experience an effective index which is, in general, different for each polarization:       n    TE    eff    ≠            n      TM      eff        .  An ideal polarization insensitive waveguide has       n    TE    eff    =            n      TM      eff        .  This can be accomplished in waveguides with cross-sections that have at least four-fold rotational symmetry so that the TE and TM modes are degenerate. A round optical fiber or a planar waveguide with a square cross-section are examples.
Polarization control becomes more challenging when features are added to the waveguide. For example, periodic structures, such as gratings, can be introduced into a waveguide.
In planar waveguide systems, gratings are typically etched into the top of the waveguide. This process results in distinctive external features called grating teeth that have the effect of creating a periodic variation in the effective refractive index of the waveguide. Gratings are commonly used in wavelength division multiplexing (WDM) systems as channel filters to isolate a particular optical channel, for example, by reflecting waves within a frequency band, called the “stopband”.
To obtain polarization insensitivity, a grating must have the identical effect on the TE and TM polarization modes. This characteristic, however, is generally difficult to achieve in monolithically fabricated planar waveguide systems, since the grating is typically not fabricated on all four sides of the waveguide due to inherent limitations in the fabrication process. This breaks the waveguide's four-fold symmetry.
Grating strength, for example, is a characteristic that the planar waveguide system designer must typically balance for each polarization mode. The grating strength, κ, is related to the modal overlap between the propagating field and the grating. Because fields for the polarization modes usually have different cross-sectional shapes, neff and κ are different for the polarization modes in asymmetric waveguides, which leads to different behavior for the polarization modes.
A number of methods have been proposed to control or reduce polarization sensitivity in grated planar waveguides when four-fold symmetry is not present. These prior art techniques for polarization control, however, suffer from a number of drawbacks. In some instances, they rely on material systems with low refractive index contrast, which is defined as the difference between the refractive index of the waveguide core and the index of the surrounding cladding. It is generally difficult to design physically small, commercially relevant devices using low index contrast material systems, however. In other instances, the prior art techniques can only be applied for relatively narrow bandwidths. In still other instances, the techniques rely on precise control of material stress characteristics during fabrication, which results in low device yields.